I keep having the same issue, I think. New memories will come up for me and will feel so real and them there are times where I feel like none of it happened, that I've just made it all up. Logically I know that, at least for the most part, they are real, and they explain a lot about me, etc. But sometimes I feel so disconnected from all of them. I spent a large part of my life denying any trauma in my past, so I think it's connected to the parts of me that still have a hard time accepting. It's almost like without the emotional connection inside me, I can't believe it.
For me, like claritytoo said, allowing those parts the needed space to have their experience, too. And understanding that dissociation is more likely the reason for that lack of connection, as opposed to it not happening, kind of reassures me in itself.
Sorry if that wasn't much help. Just wanted to let you know you're not alone in that feeling. Self compassion can be powerful if we can get there